Problem: Find the missing fraction.
Answer: Let's look at the multiples of each denominator and see which multiples they have in common. Denominator Multiples ${3}$ $3,6, 9, \underline{{12}}, 15$ ${4}$ $4, 8, \underline{{12}}, 16$ The least common multiple is ${12}$. Let's use multiplication to make each fraction have a denominator of $12$. ${\dfrac{1}{3}}=\dfrac{{1} \times 4}{{3} \times 4} = {\dfrac{4}{12}}$ ${\dfrac{3}{4}}=\dfrac{{3} \times 3}{{4} \times 3} = {\dfrac{9}{12}}$ So, $\boxed{\phantom{\dfrac14}} + {\dfrac4{12}}= {\dfrac{9}{12}}$ $\boxed{\phantom{\dfrac{5}{12}}}$ $\dfrac{4}{12}$ ${\dfrac{9}{12}}$ $+$ $\boxed{{\dfrac5{12}}} + {\dfrac4{12}}= {\dfrac{9}{12}}$ $\dfrac{5}{12}$ $\dfrac{4}{12}$ ${\dfrac{9}{12}}$ $+$ $\boxed{{\dfrac{5}{12}}} + {\dfrac{1}{3}} = {\dfrac{3}{4}}$